Through the years a number of recording devices have been created which provide a graphic output under computer control. Such devices include both graphic recorders and photoplotters in which a moveable recording element carriage is moved back and forth along a first path while a recording media is moved back and forth along a second path. In most instances, the motion paths for the media and the recording element carriage are perpendicular to each other.
In graphic recorders, one or more recording elements such as pens, are carried by the carriage and supported above the media. Means are provided for lowering one of the pens into contact with the media during times in which recording upon the media is desired. In photoplotters, the media used is a photosensitive material and hhe recording element comprises a focussed light source directed toward the media.
In either case, the recording operation takes place in response to computer control which operates the recording element carriage and media movement in a coordinated fashion to produce the desired recorded image upon media. In most systems, the recording element carriage is supported by one or more roller and track mechanisms which permit the free movement of the carriage along the carriage path. Usually, the recording element carriage is coupled to a continuous band drive systmm in which a pair of opposed rollers are supported at each end of the carriage path and a flexible band encircles the rollers and is supported in tension. At least one of the rollers is coupled to a drive motor which provides movement of the carriage band which in turn moves the carriage along the carriage path.
Because the control of carriage motion by the computer drive system requires that the computer be given precise information as to the position of the recording element carriage at all times, various devices are used to provide carriage position information to the computer. Accuracy of this information is essential for the computer to properly coordinate the relative motion between the recording element carriage and the media. While a number of different systems have been utilized to provide this information, all must provide two basic information inputs for the proper operation of the computer control system. The first is that of relative movement of the pen carriage and the second is an absolute or reference point. One of the most commonly used systems to provide relative information is known as an encoder system in which an encoder element is coupled to the motor drive system and provides a series of output pulse signals in direct correspondence to the operation of the drive system such that incremental motion information is available to the computer control. While various mechanical and electromechanical arrangements have been utilized through the years, one of the more popular encoders uses a roller or disk having a plurality of regularly spaced interleaved markings of opposite optical character, such as reflective and nonreflective which is coupled directly to the drive system. A light source is directed at the optical encoder disk and the reflected light information from the encoder disk is received by a light detecting device, which in turn produces digital information signals for communication to the computer control system. In its normal operation, the motion of the drive system in either direction results in the generation of a series of pulse signals which are, in essence, counted by the computer to determine relative motion of the carriage.
As to the need for information relating to absolute or reference position, it is essential that a reference starting point be given the computer control system to facilitate its translation of the relative information provided by the encoder into an actual or absolute position of the recording element carriage at any given time. Once the computer control system has received the absolute position, it simply maintains a count of the relative motion pulse signals from the encoder to determine the instantaneous position of the carriage at any given time. Because the accuracy of the computer systems determination of recording element carriage motion is dependent upon the accuracy and precision of the initial reference position, practitioners in the art have undertaken various approaches to providing the reference information.
In the most commonly used referencing systems, the recording element carriage is caused to move to one extreme position of its travel path and to encounter a limit detecting device. Such devices are frequently referred to as limit switches and may be mechanical, magnetic or optical in their construction. Regardless of the type selected, their basic function remains the same in that they provide an output signal once the recording element carriage has reached the reference position. In mechanical limit devices, a precision switch is positioned to be tripped or triggered by the arrival of the recording element carriage. In magnetic limit devices, an electromagnetic sensing device is positioned in the path of the recording element carriage and is tripped by the change in its magnetic field resulting from the carriage arrival. Similarly, optical limit devices provide a stationary light source and light receiver positioned on opposite sides of the carriage path. Such systems utilize the interruption of transmission caused by the arrival of the recording element carriage to indicate the reference or limit condition.
While such systems have heretofore provided the necessary reference or limit information, they are frequently costly and are often unreliable. In addition, the accuracy of such limit sensing systems requires that all parts utilized be precisely controlled. The need for precision control is particularly encountered in the use of mechanical switches, in that the actual switch tripping action must be extremely precise to provide accurate limit information.
There arises therefore a need in the art for a low cost, easy to manufacture and reliable system for providing the limit switch function in moveable carriage type recording systems which also provides the accuracy required for quality recording.